Investigation of the effects of swimming training as a rehabilitation measure on performance physiological parameters in horses. Although equine swimming as a training method and for convalescence purposes has been known for a long time, there is still uncertainty about the optimal use of swimming for training purposes or for rehabilitation. The aim of this prospective, randomised controlled study was to find out whether swimming training can increase the endurance performance of horses more than riding and lunging can. In addition, it was to be clarified whether the training plan used for swimming the horses was suitable to effectively build up condition in 6 weeks. The results of this study should provide an assessment of swimming as a training option for healthy horses and evaluate a sensible training plan for swimming in order to facilitate better use of swimming as a training measure in the future. In addition, the results should provide information on whether swimming can also be used as a rehabilitation measure for horses after illness for convalescence purposes. 20 horses of the breeds warmblood (14), thoroughbred (3) and "other" (3) were selected from the patient population of the TMBZ Seehof, which were there for the purpose of rehabilitation. 10 horses received a 6-week swimming training in addition to a 6-week riding training, while the other 10 horses were lunged for 6 weeks in addition to the 6-week riding training. The riding training served to maintain basic fitness and was not increased over the duration of the study. The swimming training was increased regularly from the second week of the study, starting with single laps, so that the horses were able to swim 2 min 20 sec or 7 laps each direction at the end of the training period. The lungeing training was also increased to 2 min 20 sec. In order to record the initial performance level and the training success over the course of the training, a field level stress test was carried out on all horses at 14-day intervals by the TMBZ Seehof and the data was evaluated in the context of this study. The parameters plasma lactate concentration, heart rate and speed were measured and the results of the tests were compared with each other and between the groups. For the lactate values, blood was taken at rest and 2 min, 15 min and 30 min after the end of exercise. Heart rate was recorded for the entire duration of the exercise until 30 min after the end of the exercise using a heart rate monitor. From the measured heart rates, the mean heart rates for the individual exercise levels were calculated, as well as the maximum heart rates and the recovery times of the heart rates after the end of the exercise. The speed was measured by stopwatch and additionally by GPS signal on the pulse belt. In the course of the training, the plasma lactate concentrations in the performed step exercise tests in the exercise blood sample (p01) decreased significantly in the swimming group compared to the lunging group. No significant difference could be shown in the mean heart rates between the two groups, nor in the recovery times of the heart rates. Reasons for this may be the small sample number and the very large scale of the data due to the difficult standardisation of the exercise tests and the large heterogeneity of the two groups. The resting lactate values and the maximum heart rates were not subject to any change in either group, as is also described in the literature. The results of the step load tests prove an effective increase of the endurance performance of the horses in the swimming group by an increase of the aerobic capacities, which could not be proven in the lunging group. This can be seen as an indication that the training plan for swimming, which is shorter and less intensive than in previous studies, can nevertheless increase the condition of the horses. The training plan used here is therefore suitable for warm-blooded horses as well as for thoroughbreds or horses of other breeds. It can therefore be better transferred into practice than the plans in previous studies, which were mainly carried out with racehorses. The horses in this study were ridden recreationally or at A-level eventing at the start of the study. Compared to other studies conducted with high-performance racehorses in full training, it can therefore be assumed that the swimming training plan in this study, which is comparatively easy at the beginning, is also suitable for horses that are in the rehabilitation phase after a long period of stall rest. The exact effects could be the subject of further investigation. Since swimming does not put any strain on the limbs due to the complete weightlessness, the horses can move freely in the pool without risk of injury and the cardiovascular system is trained in addition to building up muscles, swimming could also be suitable for rehabilitation patients for this very reason. By relieving the injured limb, it can heal and still maintain or build up condition, which is not possible through weight bearing exercise. The extent to which swimming can shorten the convalescence period was not the subject of this study and should be part of further research.